1. Field of the Invention
This invention relates to the absorption of acetylenic and multi-ene containing chemicals from an ethylene or propylene containing stream with conversion to more saturated analogs by catalytic hydrogenation. More particularly, this invention relates to a method for converting alkynes and diolefins alkenes by catalytic hydrogenation in a solvent over a fixed bed comprising a supported catalyst.
2. Description of the Related Art
Numerous patents teach hydrogenation of acetylenic and/or diolefinic compounds in the gas phase. U.S. Pat. No. 5,587,348 describes hydrogenation of C2 to C10 alkynes contained in comparable streams of like alkenes over a supported palladium catalyst containing fluoride and at least one alkali metal. Examples show hydrogenation of low concentrations of acetylene, below 1%, converted to ethylene in a gas comprising methane and ethylene at 200 psig (about 13.8 barg) and 130° F. (54° C.) and 180° F. (82° C.). Care was taken to avoid heating the gas to a runaway temperature, wherein at least 4.5% of the ethylene would be converted to ethane and the temperature would become uncontrollable, which varied from about 70° (21° C.) to 100° F. (38° C.) above the minimum temperature that would reduce the acetylene concentration to acceptable levels.
U.S. Pat. No. 6,127,310 by Brown et al. teaches that the selective hydrogenation of alkynes, which frequently are present in small amounts in alkene-containing streams (e.g., acetylene contained in ethylene streams from thermal alkane crackers), is commercially carried out in the presence of supported palladium catalysts. In the case of the selective hydrogenation of acetylene to ethylene, preferably an alumina-supported palladium/silver catalyst in accordance with the disclosure in U.S. Pat. No. 4,404,124 and its division U.S. Pat. No. 4,484,015, is used. The operating temperature for this hydrogenation process is selected such that essentially all acetylene is hydrogenated to ethylene (and thus removed from the feed stream) while only an insignificant amount of ethylene is hydrogenated to ethane (to minimize ethylene losses and to avoid a runaway reaction which is difficult to control, as has been pointed out in the above-identified patents).
U.S. Pat. No. 6,350,717 describes use of a palladium-silver supported catalyst to hydrogenate acetylene to ethylene in the gas phase and propyne and/or propadiene to propylene in the gas or mixed phase. The acetylene is present at levels of 1% in a stream of ethylene.
U.S. Pat. No. 5,856,262 describes use of a palladium catalyst supported on potassium doped silica wherein acetylene ranging in concentration from 0.01% to 5% in blends of ethylene and ethane is converted to ethylene in the gas phase.
U.S. Pat. No. 6,509,292 describes use of a palladium-gold catalyst wherein acetylene contained in a stream of principally ethylene, hydrogen, methane, ethane and minor amounts of carbon monoxide is converted to ethylene in the gas phase. This patent describes two types of acetylene hydrogenation—front end, where all the gas produced is exposed to the catalyst and tail end, where the components are first separated and a concentrated stream of acetylene or acetylenic compounds is hydrogenated.
U.S. Pat. No. 6,578,378 describes a complex process for purification of ethylene produced from pyrolysis of hydrocarbons. At the top of the de-ethanizer the vapor of the column distillate is treated directly in an acetylene hydrogenation reactor, the effluent containing virtually no acetylene being separated by a distillation column called a de-methanizer, into ethylene- and ethane-enriched tail product. The vapor containing acetylene is exposed to selective hydrogenation to reduce acetylene content of the principally ethylene gas or treated with solvent to remove it and preserve it as a separate product. In all cases the acetylene content of the pyrolysis gas contained less than 1.5 mol % acetylene.
Several patents teach hydrogenation of acetylenic compounds in the liquid phase. U.S. Pat. No. 3,755,488 describes the use of an aprotic solvent, including DMF, NMP and sulfolane, used to absorb acetylene preferentially from a gas stream composed primarily of ethylene and thereafter to subject that solvent containing absorbed acetylene to hydrogenation in the liquid phase. Selectivity is shown as high as 95% and conversion is as high as 93% when using a 5% Pd on alumina catalyst. This reaction ran for approximately 6 minutes using DMF as the aprotic solvent. The short useful reaction times of the catalyst that contained a large quantity of expensive palladium present serious impediments to industrial application.
U.S. Pat. No. 4,128,595 presents a process wherein gaseous acetylene or acetylene group containing compounds are contacted with hydrogen via an inert saturated liquid hydrocarbon stream with hydrogenation occurring over a typical Group VIII metal supported on a catalyst medium. Examples of inert saturated hydrocarbons include various hexanes, decanes and decalin. The process requires the acetylene containing compound and saturated hydrocarbon solvent be fed co-currently into the top of a trickle bed reactor because the solubility of the acetylene-containing compound in the saturated hydrocarbon solvent is poor at reaction conditions. This patent reveals that use of a polar solvent in place of the saturated hydrocarbon will provide inferior results. They show that selectivity in DMF is about 75% and that conversion dropped from 100% to 50% after operation of the reactor for 17 hours.
U.S. Pat. No. 4,571,442 relates to a process whereby acetylene and ethylene are concurrently subjected to hydrogenation over a palladium on alumina catalyst while in the presence of some liquid. The liquid comprises aromatic hydrocarbon and either primary or secondary amine. The purported advantages of using a solvent include better control of the heat released by reaction, improvement of hydrogenation selectivity and improvement of catalyst activity and stability. The results clearly show that hydrogenation of the entire stream results in loss of a substantial amount of the olefin to alkane, although using a liquid to moderate the heat released of hydrogenation may be of interest. Catalyst lifetimes are reportedly 50 hours or more. Disadvantages of using such a solvent mixture include, 1) maintaining the solvent mixture at the desired composition, 2) removal of contaminants derived from the amines, 3) the need to move a large quantity of liquid through the reactor with respect to the quantity of acetylenic and/or diolefinic contaminants contained within the olefin, and 4) the inability of the liquid to separate the reactive gas component from the inactive gas components.
U.S. Pat. No. 6,395,952 describes recovery of olefins from a cracked gas stream using metallic salts and ligand. The cracked gas stream is hydrogenated prior to scrubbing to remove acetylene from the stream.
U.S. Pat. No. 5,414,170 teaches selective hydrogenation of a stream emanating from an olefin plant after operation of a de-propanizer but prior to operation of a de-ethanizer or de-methanizer. The hydrogenation is performed on the mixed-phase propane-rich ethylene stream as well as subsequently on the vapor product. The purpose of this patent is to present a method by which the acetylenes in the front end of an olefin plant process stream are hydrogenated. The propane liquids, initially separated out of the inlet process stream are used later to cool and wash the product of the acetylene hydrogenation reactor by adding them to the acetylene-containing stream during hydrogenation.
French Patent FR 2,525,210 reveals a method for purification of a stream consisting mostly of ethylene with a smaller amount of acetylene contaminant wherein the acetylene is not converted to ethane. The basic concept is to hydrogenate a gas stream short of complete conversion, leaving some acetylene in the gas stream, then absorbing the acetylene in a solvent that extracts the acetylene from the ethylene stream. This extracted acetylene is recycled to the ethylene stream for hydrogenation. Increased conversion to ethylene is claimed.
U.S. Pat. No. 5,847,250 relates to a supported palladium catalyst employing a “promoter” from Groups I or II and the palladium being supported in silica that has been pretreated to contain the promoter. The advantage is that no carbon monoxide is needed to provide increased selectivity because the “selectivity increasing effect of the carbon monoxide is strongly temperature dependent. Large temperature gradients in the catalyst bed therefore have an adverse effect of the selectivity.” The reaction is performed in the gas phase in one or more beds with or without intermediate cooling or hydrogen gas addition. Acetylene content ranges from 0.01% to 5%. Reported selectivity ranges from 19 to 56%.
U.S. Pat. No. 4,705,906 presents a new catalyst formulation wherein acetylene is converted by hydrogenation to ethylene in the presence of CO which is in concentrations greater than 1 vol % in a temperature range between 100° C. to 500° C. The catalyst is a zinc oxide or sulfide, which may incorporate chromium, thorium or gallium oxide.
Most providers of acetylenic and diolefinic compound hydrogenation technology, such as Chevron-Phillips, utilize gas phase hydrogenation only. This can be performed directly on the cracked gas, or applied after various clean-up operations are performed (i.e., remove sulfur contaminants) or various gas components (methane, ethane) are separated out. Treating a large stream of gas requires a large reactor and very high catalyst selectivity, since the concentration of the acetylenic and diolefinic compounds is typically 1% or less. Since such catalyst is typically expensive, the conversion needs to be very complete to ensure removal of the undesirable acetylenic and olefinic compounds.
Another common technology is to use a solvent that selectively removes acetylenic and olefinic compounds from the alkene stream, such as Linde AG's DMF absorption process. The acetylenic and diolefinic compounds are not normally hydrogenated because hydrogenation of a concentrated stream of acetylenic and olefinic compounds leads to thermal runaway due to the highly exothermic nature of the hydrogenation reaction in the gas phase.
Other providers, such as Technip, provide both services. However, no commercial process provides absorption coupled with hydrogenation that can be performed in a commercially and economically viable manner as is revealed here.
Although some prior art discloses methods for separating highly unsaturated contaminants from olefinic streams and describes various means and methods for hydrogenating those contaminants to olefins as well as controlling the temperature of the hydrogenation reaction, no economical and integrated method is presently known in the art for the separation of the contaminants from an olefin stream, sequestering these contaminants selectively in an absorbent, hydrogenating those contaminants to olefins while in the absorbent using a catalyst capable of high conversion and selectively for many hours, separation of the hydrogenated compounds from the solvent by ordinary means, and recovery of the hydrogenated product for subsequent use or preferentially recombination with the initially decontaminated olefin stream. The process of the present invention can be performed in a practical and economic manner and overcomes the problems of the prior art.